Calculate standard deviation

I came across this question and  I do not know how to make the calculation. I know the values of sigma. The right answer is 0.4. This is question from Scordo

 A process in a mobile phone manufacturing project has an upper control limit of 0.12. What is the standard deviation of the process if the upper control limit is set at 3 Sigma?

  1. 0.5

  2. 0.4

  3. 0.12

  4. 0.36 

3sigma =0.12

Hence sigma (SD) = 0.04,

one point decimal error in question, such errors I observed in real exam also. but very very rare, Be careful.

 Hello sspawar,

just wanted to clarify: do you assume that lower control limit is 0.00?

Have you used the below formula?


Sigma = 0.12 /3

Control limits are generally +-3sigma (PMBOK pg196)

Here upper control limit is .12 i.e. 3sigma = .12

=> sigma (Standard deviation) = .12/3 = .04

Rajesh has clearified, there is nothing like 0, as LCL(lower control limit).

if SD is 0.04

LCL will be   -3sigma =  -0.12

UCL will be   +3sigma =  +0.12

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thanks for the clarification. U guys cleard the doubts